Shailene Woodley and Ansel Elgort may be leading The Fault in Our Stars, but supporting the love story of Hazel Grace Lancaster and Augustus Waters -- and constantly cutting their tear-jerking tension with hilarious one-liners -- is fellow terminally ill teen Isaac, played by Nat Wolff.

"This is probably the hardest character I've played because it seems so simple," the 19-year-old actor tells The Hollywood Reporter of Isaac, the one-eyed, lovestruck best friend of Augustus who loses his sight completely in the film. "How am I gonna make it so he's a super-funny character who's not the butt of the joke? I met with a lot of blind people, and they said, they're very offended by [portrayals of blind people onscreen] -- people who are walking into stuff, making fun of them.

"So what I tried to do was find the edge behind the humor," he says of his character's sarcastic jabs and one-liners -- most of which, director Josh Boone says, were completely improvised, especially throughout the bittersweet trophy-trashing scene in Augustus' basement. (Boone and Wolff previously collaborated for Stuck in Love and will do so again in The Stand.) "It's like he was using humor as a defense."

For practice, Wolff walked around his home and town blindfolded, alongside his mother, who made sure he didn't hurt himself and accompanied him to a meal at a local restaurant. And while filming, he did all he could to minimize his eyesight on set. "I didn't want to have to fake being blind if I didn't have to, so I put these two contacts into my eyes that made it so I really was blind -- they were completely black," he explains. "Once I went blind, I always had blackout contacts and black glasses so no one could see my eyes."

Wolff noted that though it took some time to adjust, "When I watched the movie, I was like, 'Why is my body language different?' It's because of my contacts, and I didn't even realize it at the time -- the initial 'bumbling' around on set was worth it. It made it so scary, and so real to the point where that eulogy scene -- I was blind during that scene -- something about it just freed me up, because I wasn't self-conscious. I didn't see the camera, I didn't see any of the people. It was nice."

Another major scene that leans on Isaac's lack of eyesight is when he eggs a sports car from across the street (pictured above) -- and ends up hitting the target repeatedly. Wolff reassures he stayed in character: "I was throwing them blind!"